Glutamate is the principal excitatory neurotransmitter at most synapses, and strongly modulates dopaminergic pathways involved in addictive behaviors. There are two different types of glutamate gated-ion channels, NMDA receptors and non-NMDA receptors. The NMDA receptors (produced by genes NR1, NR2A-NR2D) have attracted considerable attention due to its role in long-term potentiation (LTP), a proposed form of memory. NMDA receptors are also sites of action of the hallucinogenic street drug phencyclidine (PCP; "angel dust"). Non-NMDA receptors are encoded by genes GluR1-GluR7, KA-1 and KA-2, and represent the principal sites of glutamatergic transmission. In the course of molecular characterization of glutamate receptors, we have cloned four new glutamate receptor subunit variants. Subunits NR1b and NR1c define new splice variants of the NR1 subtype of glutamate receptors. Sequencing revealed that NR1b differs from NR1a by the presence of a 21 amino acid insert near its amino-terminus and by a different sequence of the carboxy-terminus. Properties of NR1b were studied in the oocyte expression system. NR1b channels exhibited a lower affinity for NMDA and glutamate, whereas affinities for PCP and glycine were nearly identical. Spermine potentiation of NR1b receptors was abolished, probably due to the amino-terminal insertion which has a net positive charge. Furthermore, NR1b channels were potentiated by protein kinase C about 20-fold, what is much less than the potentiation of NR1a channels, about 4-fold. These observations show that alternative splicing plays a role in generating NMDA receptor channels with different properties. Phosphorylation of NMDA receptors by protein kinase C regulates its channel activity and may have a role in LTP. Elucidation of the functional significance of alternately spliced NMDA receptors may have significance for interaction with drugs such as PCP. In addition, we identified two glutamate receptor subunit CDNAS termed GluR5-1d and GluR6-2 which define new isoforms of kainate receptors with altered structure of the carboxy-terminal domain. These novel subunits are apparently generated by alternative splicing of a facultative intron. The position of this intron has been predicted from the gene structure of the kainate binding protein (KBP), another glutamate receptor gene which we have characterized in detail. Genetic mapping of several glutamate receptor genes revealed that the GLUR5 gene is located on human chromosome 21 close to the familial amyotrophic lateral sclerosis locus and in the region implicated in Down's Syndrome. Physical mapping of the GLUR5 gene with yeast artificial chromosomes showed that the GLUR5 gene is located between APP and SOD1 genes at 21q22.1. The relatively large size of the GLUR5 gene, 400-500 kb, raises questions about its functional significance.